Electrolytic process for coating aluminum and aluminum alloys



Ma 16,- 1944. M FARR, R 2,34%

ELECTROLYTIC PROCESS FOR COATING ALUMINUM AND ALUMINUM ALLOYS Filed May 13, 1940 Patented 16, 1944 UNITED STATES PATENT OFFICE 2,349,083 ELECTROLYTIC raocass For. coa'rmo ALUMINUM' annoys Mi hael Farr, Jr., Columbus, Ohio, assignor to The B. 8; '1. Floor Company, Columbus, Ohio, a corporation of Ohio Application May 13, 1940, Serial No. 334,763

4 Claims. (Cl. 204-58) This invention relates -toa process whereby aluminum and aluminum alloys are anodically provided with an even, hard, corrosive-resisting oxide coating.

It is the object of this invention to provide an economical and efficient method for effecting .the formation on the outer surfaces of an aluminum or aluminum alloy body of a hard, fine grained oxide coating which, at first, is permeable and .adsorbent, adaptable for impregnation with an oil or dye and for subsequent bufllng, or may be left in its natural color and buflfed. After being buil'ed, however, the electrolytically formed coating becomes substantially impermeable and nonadsorbent. and when in this latter condition, said coating possessing its maximum hardness. In previous processes of an analogous nature. a comparatively highly concentrated electrolyte and /01' a comparatively high voltage are required to produce a'coating conforming substantially in quality to that produced by means of my process.

My invention, therefore. consists in the provision of a method wherein the aluminum or aluminum alloy to be treated is made the anode in an electrolyte of about 10% H2804, either C. P. or a commercial grade producing satisfactory results. Then a direct current of from six to twelve volts is applied. In this 10% H2504 electrolyte, usin the voltages stated. the amperage is automatically controlled, a feature not considered possible in prior art processes of which I am aware.

As an important feature of my invention, I make use of a special system of electrolysis tanks. which system contributes to the automatic controlling of current flow. In this system, I employ an outer tank, preferably of lead, and which isused as the cathode, Within this outer tank, I

demands of the coating thickness.

position a smaller and inner tank of glass, hard rubber or other suitable non-conductor. Th inner tank is supported upon several lead blocks in a manner so that the cellulartank does not actually lessen the effective cathode surface. 'The inner tank is not quite as deep as the outer tank. so that when the latter is filled to a predetermined level with the electrolyte, the inner tank is completely immersed. By this arrangement of In carrying out the process, the total'body to be coated is first cleansed of grease, dirt or foreign impurities, in any eflicient way. A practicable method is to clean the metal by subjecting the same to a bath composed of a hot solution of lye. although other cleansing agents or solvents may be used. The metal is then rinsed and suitably supprted within the confines of the inner tank, surrounded of course by the electrolyte. The lead walls or lining of the outer tank are formed to constitutethe cathode of the cell and the aluminum or aluminum alloy to be treated is connected with the source of electrical energy to form the anode. An electric current of irom six to twelve volts is turned on for about thirty minutes, the period of such electrolytic activity being varied to meet the The metal is then removed from the cell, rinsed and dried. At. this stage of procedure, the applied oxide coating is relativelyp'ervious and adsorbent, p0ssesS-' ing a light color. It may now be buffed to imparta desired lustrous appearance thereto, and after such bufilng, the coating becomes impervious and non-adsorbent, but retaining its light color. Again immediately following removal from the electrolytic cell, the coating may be impregnated with an oil or a suitable pigment and later buffed. so that various color effects may-be obtained.

In the accompanying sheet of drawing, the single figure is a diagrammatic view in vertical section, disclosing my improved tank arrangement. In the drawing, the numeral i designates the outer tank, the same being of any desired dimensions, possessing a substantially box-like order above specified.

Supported within the inner tank 4 by wires 6, or other suitable current conveying members, is the article I to be treated. In "the present instance, a piece of molding has been illustrated, th s molding being formed of an extruded aluminum alloy. The supporting wires 5 extend upwardly and are hooked or otherwise connected to a bar 8 to which current is conducted from a Positioned upon spaced lead blocks 0! dashed potential by a lead I. The negative terminal of the currentsource is connectedbyasecondlead ilwiththeleadlining 2 to complete the circuit and when current ilow is establishd by the closing of a suitable -switch. not shown. an electrolytic action is initiated by which the article I will be coated.

It will be noted that the inner tank. which has the same box-like form of the outer tank. has the upper edges of its side walls terminated just below the level of the electrolyte, so that current iiow passing through the cell is elongated and compelled to travel through the restricted porbody, wherein the coating is unusually even, hard and resistant to abrasive iorces. Moreover, the

coating does not smudge upon being handled or when in use nor lose its highly lustrous appearance.

What is claimed is:

l. A process oi producing on aluminum suriaces an electrolytic coating which comprises:

'makingthe aluminum to be coated an anode in an electrolyte containing about 10% by weight oi sulphuric acid, surrounding the aluminum anode with an inner tank composed of a dielectric with said tank completely immersed in said electrolyte. supporting said inner tank within an outer tank comprising an electrical conductor in which the electrolyte is contained at a level above that oi the inner tank, said outer tank forming the cathode connection of the electrolytic cell, applying to the electrolytic cell an electric current having a voltage oi from six to twelve volts, maintaining the electrolyteat a ment composed of a dielectric and completely immersed in said electrolyte, the inner tank element being located between the anode and the cathode, and impressing a currnt having a voltage oi from six to twelve volts on said electrolyte.

8. In electro-coating articles of aluminum and aluminum alloys, the process of making such an article an anode in sulphuric acid, containing the sulphuric acid electrolyte within an outer receptacle forming a cathode, shielding the article to be coated by surrounding the same with an inner tank element composed of a dielectric and completely immersed in said electrolyte, said inner tank element being located between the anode and the cathode, and impressing a current of from six to twelve volts on said electrolyte.

4. A method oi electro-coating articles formed of aluminum and aluminum alloys comprising passing an electric current through an electrolyte in which the article is immersed as an electrode to form an anodic him on the surface of the article surrounding the article while in said electrolyte with a dielectric shield disposed between the article and the cathode, the shield being completely immersed in said electrolyte -and being maintained at a constant value between all parts of the article and the cathode. and impresing a current oi six to twelve volts on said electrolyte. 

